The present invention is directed to an adapter for achieving a leak-free connection between a plastic pipe and a metallic conduit such as a sprinkler head.
A sprinkler system is usually made of many plastic pipes running from a source of fire retardant such as water and terminating in metallic sprinkler heads at various locations in a building. The water is maintained in the system under pressure so that when ambient temperature rises sufficiently to, for example, melt a specially formulated material blocking the end of the sprinkler head, water is released through the sprinkler head.
A sprinkler system uses an adapter for connecting the plastic pipe to the metal sprinkler head. A prior art adapter comprised an internally threaded and tapered plastic fitting. A correspondingly threaded and tapered metal sprinkler head was screwed into the threaded, tapered end of the fitting. The plastic pipe was adhesively secured in the other end of the fitting.
With this prior art adapter the threaded connection between the metal sprinkler head and the plastic fitting was subject to attack and deterioration by the adhesive used to secure the plastic pipe to the fitting. Additionally, cracks appeared in the fitting due to a wedging action of the tapered sprinkler head if over tightened in the tapered fitting bore. The deterioration and cracks resulted in water leakage from the adapter.
An attempt to address these problems resulted in another prior art adapter with which the sprinkler head is not screwed directly into the plastic fitting. Instead a metal insert is first inserted into the plastic fitting and the sprinkler head is then screwed into the metal insert. A gasket or O-ring is placed between the insert and the fitting. Such an adapter is shown in U.S. Pat. No. 4,905,766. Here, the metal insert is held in place in the plastic fitting by matching elongated teeth on the outer surface of the insert and on inner surface of the fitting. The insert can also be held in the fitting by being roll formed, that is by crimping the insert against an annulus within the bore of the fitting.
The adapter shown in U.S. Pat. No. 4,905,766 is less than satisfactory for a number of reasons. First, water leakage can still occur, particularly if the roll form step is omitted or carried out improperly. The metal insert can then be expelled from the plastic fitting when water pressure is established in the sprinkler system. Water leakage can occur even if the roll form step is carried out because this step causes an outward compression of the metal insert onto the plastic fitting, and the compression can crack the fitting.
Second, the metal insert cannot be inserted on site, making testing of the integrity of the adapter prior to establishment of the final water pressure in the sprinkler system essentially impossible. Third, the metal insert is not easily replaceable should the insert begin to leak or show other signs of deterioration.
Fourth, the gasket is not replaceable. Replacement of the gasket can become important if the gasket begins to break apart and loose its sealant capability. The gasket can begin to break apart due to the action of the adhesive used to attach the plastic pipe to the fitting and due to the effect of long term exposure of the water in the sprinkler system.
Fifth, the adapter can be expensive to make due to at least the presence of a complex set of elongated teeth on the fitting and on the insert, and by requiring a roll form step. The roll form step necessitates an additional machine or tool to prepare the adapter for use.
Thus, there is a need for an adapter that: (a) provides an essentially leak-free connection of plastic to metal even after long usage; (b) has a metal insert that can be inserted on site; (c) has a metal inset that is replaceable; (d) has a replaceable gasket; and (e) that can be made relatively inexpensively.